In a plasma processing apparatus using capacitively coupled plasma such as, for example, a parallel plate type plasma processing apparatus, a processing gas is introduced between an upper electrode and a lower electrode within a processing chamber, and the introduced processing gas is excited by an electric field so that plasma is generated (plasma is ignited).
When igniting the plasma, it is necessary to increase a voltage across an electrode (the upper electrode or the lower electrode) connected to a high frequency power supply so as to generate an electric field between the upper electrode and the lower electrode. Before the ignition of the plasma, however, since the plasma serving as a conductor does not exist between the upper electrode and the lower electrode, a high frequency current generated from the high frequency power supply 110 does not flow between the upper electrode 111 and a susceptor 112 serving as the lower electrode. Rather, the high frequency current flows from the upper electrode 111 connected to the high frequency power supply 110 to a ground via an insulation member (e.g., an insulator) of a chamber 113. In connection with this, please refer to, for example, FIG. 11.
Meanwhile, after the ignition of the plasma, the high frequency current generated from the high frequency power supply 110 flows to the ground through the space between the upper electrode 111 and the susceptor 112 since the plasma P exists between the upper electrode 111 and the susceptor 112.
That is, a first route L1 of a high frequency current (indicated by a solid line arrow in FIG. 11) before the ignition of the plasma P and a second route L2 of a high frequency current (indicated by a broken line arrow in FIG. 11) after the ignition of the plasma P are different from each other. Further, a first path capacity such as, for example, an insulator, exists in the route L1 while a path capacity of a sheath S1, a path capacity of the plasma P, a path capacity of a sheath S2, and a path capacity between a wafer W and the susceptor 112 exist in the second route L2. Thus, the path capacity of the first route L1 and the path capacity of the second route L2 are different from each other. As a result, a load impedance from the high frequency power supply 110 to the ground is greatly changed before and after the ignition of the plasma P.
When the path capacity in a route is changed, a resonance frequency in the route is changed, and further a reflection minimum frequency close to the resonance frequency is also changed. Therefore, in a case where the high frequency power supply 110 is generating a high frequency current having a reflection minimum frequency of the first route L1 before the ignition of the plasma P, when the plasma P is ignited and the route of the high frequency current is changed from the first route L1 to the second route L2, the frequency of the high frequency current is different from the reflection minimum frequency of the second route L2 and thus, most of the high frequency current is reflected to the high frequency power supply 110. As a result, the high frequency current hardly flows in the second route L2 and the electric field between the upper electrode 111 and the susceptor 112 may not be maintained so that the plasma P may not be maintained.
In connection with this, it has been proposed to change the frequency of the high frequency current generated from the high frequency power supply before and after the ignition of the plasma. In particular, since changing a capacity of a variable capacitor 115 of a matching circuit (matcher) 114 disposed between the high frequency power supply 110 and the upper electrode 111 is equivalent to changing the frequency of the high frequency current generated from the high frequency power supply 110, it has been proposed to change the capacity of the variable capacitor 115 of the matcher before and after the ignition of the plasma.
However, the change of the capacity of the variable capacitor 115 involves a mechanical operation in the variable capacitor 115 and thus requires a certain length of time. Therefore, the change of the capacity is not able to temporally follow the change from the reflection minimum frequency of the first route L1 to the reflection minimum frequency of the second route L2 before and after the ignition of the plasma. As a result, in some cases, a period where the high frequency current hardly flows in the second route L2 occurs so that the plasma P may not be maintained.
Thus, for example, Japanese Patent Laid-Open Publication No. H10-64696 proposes a technology of electronically changing the frequency of the high frequency current generated from the high frequency power supply 110, for example, a technology for controlling the frequency of the high frequency current by a controller installed separately from the high frequency power supply to increase the frequency of the high frequency current when the plasma is ignited. When electronically changing the frequency of the high frequency current, the frequency may be quickly changed. Thus, the frequency of the high frequency current may temporally follow the change from the reflection minimum frequency of the first route L1 to the reflection minimum frequency of the second route L2.